1. Highly Selective Allylation Using Pentacoordinate Organosilicates On of the most interesting features in organosilicon chemistry is that stable high coordinate species can be isolated. We have found that pentacoordinate allylsilicates and related compounds, unknown hitherto, can be readily prepared and used as nucleophiles, some of which are reported. Topics are preparation of pentacoordinate allylsilicates, substituted by alkoxy groups,which are useful and storable reagents, regio- chemo- and stereoselective allylations of carbonyl compounds without catalyst, and stereochemical outcomes of asymmetric allylation with optically active allylsilicates which are interpreted by a six-membered cyclic transition state, in sharp contrast with an anti-SE^1 mechanism for the allylation using tetracoordinate allylsilane.2 Novel Synthesis of Alkylideneazomethine Yilds and Related 1,3-Dipolar Reagents and Synthetic Application to Organic Synthesis^2 Azomethine ylids and related 1,3-dipolar reagents are important and interesting chemical species from both synthetic and theoretical points of view. These species are generally unstable and inaccessible if the dipolar centers are not stabilized by electron-withdrawing or conjugating groups. We report here that a variety of N-silylmethyl-substituted ketene N,S-acetals react smoothly with activated alkenes, carbonyl compounds, and thioketones in the presence of cesium fluoride, to afford alpha-alkylidenepyrrolidines, oxazolidines, and thiazolidines, via a 1,3-elimination of methylthiotrimethylsilane. Similarly, N-silylmethyl- substituted isothioureas react with carbonyl and thiocarbonyl compounds promoted by fluoride ion to give the corresponding [3 + 2] cycloadduct, 2-imino-1,3-oxazolidines and 2-imino-1,3-thiazolidines in good yield. These reagents can be viewed as synthetic equivalents of alkylideneazomethine ylids and iminoazomethine ylids.